A standardized pathology protocol for use in carcinogenesis bioassay was developed and implemented in the NCI Bioassay segment program. This was to assure uniform and thorough pathologic evaluation of animals under test, and address the minimal levels of gross and microscopic examinations as well as guidelines for determining necessary facilities, technical methods and personnel qualifications. A three day workshop was organized and held to arrive at nomenclature and definitions of specific rat liver tumors and related lesions. Twenty pathologists with wide and varied experience were convened for 3 days and discussed selected histologic slides. A consensus was reached on all lesions, the most significant being a recommendation of the term "neoplastic nodule" for lesions diagnosed as hyperplastic nodules. This decision was based on all available evidence relating to the biologic nature of the nodules.